U.S. Pat. No. 5,798,521 to Froggatt, discloses an apparatus and method for measuring strain of gratings written into an optical fiber. By measuring the complete spectral response of the Bragg grating, the strain at each point in the grating can be measured. The apparatus comprises a control system, a data acquisition (DAQ) circuit, detectors, a laser controller, and a coherent light source or tunable laser. The reference and measurement fringes are detected and sampled such that each sampled value of the reference and measurement fringes is associated with a corresponding sample number. The wavelength change of the laser, for each sample number, is determined by processing the signal from the reference optical fiber interferometer. Each determined wavelength change is matched with a corresponding sampled value of each measurement fringe. Each sampled measurement fringe of each wavelength sweep is transformed into a spatial domain waveform. The spatial domain waveforms are summed to form a summation spatial domain waveform that is used to determine location of each grating with respect to a reference reflector. A portion of each spatial domain waveform that corresponds to a particular grating is determined and transformed into a corresponding frequency spectrum representation. The strain on the grating at each wavelength of optical radiation is determined by determining the difference between the current wavelength and an earlier, zero-strain wavelength measurement. For this application, the measurement of the instantaneous wavelength of the tunable laser by the single reference interferometer is sufficiently accurate.
U.S. Pat. No. 6,376,830 to Froggatt et al. is directed toward a system and method for measuring the transfer function of a guided wave device. In particular, the N×N scalar transfer function elements for an N-port guided wave device are measured. Optical energy of a selected wavelength is generated at a source and directed along N reference optical paths having N reference path lengths. Each reference optical path terminates in one of N detectors such that N reference signals are produced at the N detectors. The reference signals are indicative of amplitude, phase and frequency of the optical energy carried along the N reference optical paths. The optical energy from the source is also directed to the N-ports of the guided wave device and then on to each of the N detectors such that N measurement optical paths are defined between the source and each of the N detectors. A portion of the optical energy is modified in terms of at least one of the amplitude and phase to produce N modified signals at each of the N detectors. At each of the N detectors, each of the N modified signals is combined with a corresponding one of the N reference signals to produce corresponding N combined signals at each of the N detectors. A total of N2 measurement signals are generated by the N detectors. Each of the N2 measurement signals is sampled at a wave number increment Δk so that N2 sampled signals are produced. The N×N transfer function elements are generated using the N2 sampled signals. Reference and measurement path length constraints are defined such that the N combined signals at each of the N detectors are spatially separated from one another in the time domain.
Because U.S. Pat. No. 6,376,830 is directed toward optical instrumentation, the accuracy requirements on the phase measurements are quite severe. The measurement is based on the assumption that a long interferometer can be used to precisely monitor the instantaneous wavelength of a tunable laser. This assumption is valid under most regimes of operation, and to the degree-of-accuracy generally required. However, when lasers with significant tuning-speed variations are used coupled with long (>20 m) paths and a requirement of phase accuracies on the order of milli-radians, then, the assumption is no longer valid (as is the case with optical instrumentation). The breakdown of the assumption occurs when the timescale of the tuning speed variation occurs on the time scale of the delay in the interferometer. In turn, it becomes necessary to correct for errors generated by the optical source.
U.S. Pat. No. 6,566,648 to Froggatt describes an apparatus and method for measuring strain of gratings written into an optical fiber. Optical radiation is transmitted over one or more contiguous predetermined wavelength ranges into a reference optical fiber network and an optical fiber network under test to produce a plurality of reference interference fringes and measurement interference fringes, respectively. The reference and measurement fringes are detected, and the reference fringes trigger the sampling of the measurement fringes. This results in the measurement fringes being sampled at 2Π increments of the reference fringes. Each sampled measurement fringe of each wavelength sweep is transformed into a spatial domain waveform. The spatial domain waveforms are summed to form a summation spatial domain waveform that is used to determine location of each grating with respect to a reference reflector. A portion of each spatial domain waveform that corresponds to a particular grating is determined and transformed into a corresponding frequency spectrum representation. The strain on the grating at each wavelength of optical radiation is determined by determining the difference between the current wavelength and an earlier, zero-strain wavelength measurement. The apparatus and method disclosed herein fails to disclose an auxiliary interferometer that corrects for residual errors resulting from the laser.
An object of the present invention is to provide an apparatus and method for correcting for errors generated by a laser with non-ideal tuning characteristics.
Another object of the present invention is to provide an apparatus and method for correcting errors generated by a laser with non-ideal tuning characteristics that employs at least one auxiliary interferometer in parallel with a sampling interferometer and a measurement interferometer.